Anisotropic magnetic flakes

ABSTRACT

The invention relates to anisotropic, reflective, magnetic flakes. In a liquid carrier and under influence of an external magnetic field, the flakes attract to one another side-by-side and form ribbons which provide higher reflectivity to a coating and may be used as a security feature for authentication of an object.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/233,667, filed Sep. 19, 2008, (now U.S. Pat. No. 9,662,925) whichclaims priority to U.S. Provisional Patent Application No. 60/973,546filed Sep. 19, 2007. U.S. patent application Ser. No. 12/233,667 is alsoa continuation-in-part of U.S. patent application Ser. No. 12/051,164,filed Mar. 19, 2008, which claims priority to U.S. ProvisionalApplication No. 60/919,204, filed on Mar. 21, 2007. U.S. patentapplication Ser. No. 12/233,667 is also a continuation-in-part of U.S.patent application Ser. No. 12/107,152, filed Apr. 22, 2008, whichclaims priority to U.S. Provisional Application No. 60/913,423, filedApr. 23, 2007. U.S. patent application Ser. No. 12/233,667 is also acontinuation-in-part of U.S. patent application Ser. No. 11/461,870,filed Aug. 2, 2006, (now U.S. Pat. No. 7,625,632) which is acontinuation-in-part of U.S. patent application Ser. No. 11/028,819,filed on Jan. 4, 2005, (now U.S. Pat. No. 7,300,695) which is adivisional application of U.S. patent application Ser. No. 10/243,111,filed on Sep. 13, 2002, (now U.S. Pat. No. 6,902,807). U.S. patentapplication Ser. No. 11/461,870 is also a continuation-in-part of U.S.patent application Ser. No. 11/313,165, filed Dec. 20, 2005, (now U.S.Pat. No. 7,604,855) which is a continuation-in-part of U.S. patentapplication Ser. No. 11/022,106, filed on Dec. 22, 2004, (now U.S. Pat.No. 7,517,578) which is a continuation-in-part of U.S. patentapplication Ser. No. 10/386,894, filed Mar. 11, 2003, (now U.S. Pat. No.7,047,883) which claims priority to U.S. Provisional Patent ApplicationNo. 60/410,546 filed Sep. 13, 2002, U.S. Provisional Patent ApplicationNo. 60/410,547 filed Sep. 13, 2002, and U.S. Provisional PatentApplication No. 60/396,210 filed Jul. 15, 2002. U.S. patent applicationSer. No. 11/461,870 also claims priority to U.S. Provisional PatentApplication No. 60/713,127 filed Aug. 31, 2005, all of the aboveapplications of which are incorporated herein by reference.

The Ser. No. 11/461,870 patent application is related to commonly ownedU.S. patent application Ser. No. 10/029,405, filed Dec. 20, 2001, (nowU.S. Pat. No. 6,749,936); is related to commonly owned U.S. patentapplication Ser. No. 09/919,346, filed Jul. 31, 2001, (now U.S. Pat. No.6,692,830); and is related to commonly owned U.S. patent applicationSer. No. 10/117,307, filed Apr. 5, 2002, (now U.S. Pat. No. 6,841,238)the disclosures of which are hereby incorporated in their entirety forall purposes.

TECHNICAL FIELD

The present invention relates generally to thin pigment flakes, and moreparticularly to providing alignment of thin magnetic flakes in anexternal magnetic field.

BACKGROUND OF THE INVENTION

Reflective metallic and color-shifting flakes are used in reflective andcolor-shifting inks or paints. Images printed with the inks or coatingsmade using the paints have their reflective or color parameters worsethan those of a solitary flake due to the gaps between adjacent flakesfilled with a less reflective carrier. An increase of pigmentconcentration can improve the reflectivity of the printed images andpaint coatings, but is associated with additional cost, with thickercoatings, and with flakes overlapping one another preventing them fromlying flat against the substrate.

Accordingly, it is an object of the instant invention to provide a costeffective highly reflective coating containing reflective orcolor-shifting flakes. It is another object of the invention to provideflakes for such coatings or inks, as well as a method of manufacturingthe flakes enabling said coatings. It is yet another object of theinvention to provide a method of authentication of an object having thereflective coating thereon.

SUMMARY OF THE INVENTION

The present invention relates to an anisotropic magnetic flake forforming a reflective coating. The flake has a layered structure having athickness in the range of 50 nm-10 microns and a two-dimensional shapewith a longest planar dimension in the range of 1-500 microns. Thelayered structure includes a magnetic layer for aligning the flakesubstantially parallel to the surface of the reflective coating, whensaid flake is disposed in a liquid carrier under influence of anexternal magnetic field. The magnetic layer has a structure whichprovides in-plane magnetic anisotropy forming an angle of at least 20degrees with the longest planar dimension. Optionally, the layeredstructure first and second reflector layers having reflectivity ofgreater than 50%, for providing reflectivity to the reflective coating;wherein the magnetic layer is hidden between the reflector layers.

One aspect of the invention relates to the described above flakes havinga specific two-dimensional shape particularly well suited for makingribbons. The shape of the flakes has two sides substantially parallel toeach other. The magnetic layer is formed so as to have an in-planemagnetic anisotropy substantially orthogonal to the two sides foralignment of the flake side-by-side with one or more flakes of a samestructure when said flakes are disposed in a liquid carrier underinfluence of an external magnetic field while forming the reflectivecoating. The flakes have no grating thereon for providing high specularreflectivity. Preferably, the flakes have a square shape.

Another aspect of the present invention relates to a method ofmanufacturing the flakes wherein the anisotropy of the magnetic layer issubstantially orthogonal to two sides of the two-dimensional shape ofthe flake. The method includes the steps of: (a) providing a substratefor supporting a releasable coating including the first and secondreflector layers and the magnetic layer formed of a magnetic material;(b) embossing or etching the substrate with a plurality of frames havingthe two-dimensional shape with the two sides substantially orthogonal toa first direction, before or after the releasable coating is applied tothe substrate; (c) coating the substrate with the releasable coating soas to provide the magnetic layer having a magnetic anisotropy in thefirst direction, wherein the releasable coating upon removal from thesubstrate breaks apart into the flakes; and, (d) removing the releasablecoating from the substrate and breaking it into the flakes. In oneembodiment of the method, the magnetic layer is deposited using twosources which provide the magnetic material to a same portion of thesubstrate from different angles.

Yet another aspect of the present invention relates to a coatingincluding a solidified carrier and a plurality of flakes dispersedtherein. All the flakes have a same two-dimensional shape, such that twosides thereof are substantially parallel to each other, and have themagnetic layer such that the in-plane magnetic anisotropy issubstantially orthogonal to said two sides. A portion of the flakesforms a ribbon of at least three flakes adjacent to one another so as tobe side-by-side with gaps between the flakes of no greater than 500 nm,when said flakes are disposed in a liquid carrier under influence of anexternal magnetic field while forming the coating.

The ribbons of three or more flakes may be used as a security feature ondocuments, banknotes, etc. The instant invention provides a method ofauthentication of an object including the step of identifying a ribbonin a flake-containing coating. In one embodiment, the flakes forming theribbon have a binary grating thereon, which provides a pattern ofreflected light beams used for authentication of the object.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1A is a simplified plan view of a conventional magnetic flake in anexternal magnetic field, according to prior art;

FIG. 1B is a simplified plan view of a chain formed of magnetic flakesshown in FIG. 1A, in an external magnetic field;

FIG. 2 is a simplified plan view of a ribbon formed by anisotropicmagnetic flakes of the instant invention in an external magnetic field;

FIG. 3A is a perspective view of a substrate used for manufacturinganisotropic magnetic flakes of the instant invention;

FIG. 3B is a flow chart of a method of flake manufacturing in accordancewith the instant invention;

FIG. 4A is an illustration of a magnetic material deposition accordingto one embodiment of the instant invention;

FIG. 4B is a plan view of the substrate shown in FIG. 4A;

FIG. 5A is a side view illustration of a magnetic material depositionaccording to one embodiment of the instant invention;

FIG. 5B is a plan view of the substrate shown in FIG. 5A;

FIG. 6A is a side view illustration of a magnetic material depositionaccording to one embodiment of the instant invention;

FIG. 6B is a plan view of the substrate shown in FIG. 6A;

FIG. 7 is a side view illustration of a magnetic material depositionaccording to one embodiment of the instant invention;

FIG. 8A is a microscopic image of randomly aligned conventional flakesin absence of a magnetic field;

FIG. 8B is a microscopic image of conventional flakes in a magneticfield;

FIG. 8C is a microscopic image of anisotropic magnetic flakes of theinstant invention in a magnetic field; concentration of the flakes in acarrier is 10 wt %;

FIG. 8D is a microscopic image of anisotropic magnetic flakes of theinstant invention forming ribbons in a magnetic field;

FIG. 8E is a microscopic image of anisotropic magnetic flakes of theinstant invention forming a highly reflective coating in a magneticfield;

FIG. 9A is a simplified plan view of shaped flakes;

FIG. 9B is a simplified plan view of a ribbon formed by flakes of theinstant invention with a hydrophobic coating;

FIG. 9C is a simplified plan view of ribbons formed by flakes of theinstant invention without a hydrophobic coating;

FIGS. 10 and 11 are simplified cross sections of flakes according toembodiments of the present invention;

FIG. 12 is a cross section of a magnetic flake with a reflectivegrating;

FIG. 13 is a microphotograph of ribbon-like structures; and

FIG. 14 is an illustration of a method of object authentication.

DETAILED DESCRIPTION

A magnetic flake is a pigment flake that includes a magnetic material.It is known that a square-shaped magnetic flake without a grating hasits easy magnetic axis, i.e. a direction of its magnetic moment, along adiagonal of the square, and North and South magnetic poles—at oppositecorners of the square. FIG. 1A, which is a copy of FIG. 2a from U.S.Pat. No. 7,300,695 issued Nov. 27, 2007, to Argoitia et al., illustratesa rectangular magnetic flake 40 with sides 42 and 44 in a liquid mediumand under influence of a magnetic field. The flake 40 orients so as tohave a diagonal along the direction of the applied magnetic field 46.North and South magnetic poles of different flakes attract and flakesmay form corner-to-corner chains such as shown in FIG. 1B.

It has been unexpectedly discovered that, when dispersed in a liquidcarrier and impacted by a magnetic field, square-shaped, non-gratedmagnetic flakes of a particular kind form different structures, namelyribbons. With reference to FIG. 2 flakes 47 within the ribbons 48 areadjacent to each other side-by-side as opposite to the corner-to-corneradjacency shown in FIG. 1B. The side-by-side attraction of two flakes 47manifests the location of magnetic poles at two opposite sides of thesquare flake 47 and the direction of a magnetic moment parallel to twoother sides of the square 47. The square flakes 47 orient in a magneticfield 46 so as to align a side of a flake 47 along the lines of themagnetic field 46.

In general terms, conventional non-diffractive flakes have easy axesalong their longest planar dimensions, whereas the flake of the instantinvention has an easy axis, or magnetic anisotropy, at an angle with thelongest planar dimension.

The unexpected effect may be attributed to magnetic anisotropy caused bya method of flake manufacturing. Possible types of magnetic anisotropy,such as magneto-crystalline anisotropy, stress-induced magneticanisotropy, and magnetic anisotropy induced by the substrate topography,are described in “Hitchhiker's Guide to Magnetism,” Bruce M. Moskowitz,Environmental Magnetism Workshop, 5-8 Jun. 1991.

With reference to FIG. 3B, the method of flake manufacturing includes asubstrate-providing step 510 wherein a substrate is provided forsupporting a releasable coating. In a substrate embossing/etching step520, the substrate, optionally with a coating thereon, is provided witha plurality of frames embossed or etched onto the substrate, for exampleas taught in U.S. Pat. No. 6,902,807 issued Jun. 7, 2005, and US PatentApplication No. 20080107856 published May 8, 2008, both to Argoitia etal. and incorporated herein by reference. The shape and orientation ofthe frames is related to the direction of the anisotropy so that thetwo-dimensional shape of a frame has two sides substantially parallel toeach other and substantially orthogonal to the desired anisotropydirection.

Then, in a coating step 530, the substrate is coated with the releasablecoating as disclosed in U.S. Pat. No. 6,838,166 issued Jan. 4, 2005, andU.S. Pat. No. 6,808,806 issued Oct. 26, 2004, both to Phillips et al.and incorporated herein by reference. The releasable coating includesthe first and second reflector layers and a magnetic layer formed of amagnetic material. The magnetic layer is deposited so as to have amagnetic anisotropy in a desired direction. Then, in a coating removingstep 540, the releasable coating is removed from the substrate andbroken apart providing anisotropic magnetic flakes of the instantinvention.

In one embodiment of the method, a substrate provided in thesubstrate-providing step 510 is patterned in the substrateembossing/etching step 520 following the step 510. With reference to inFIG. 3A, a substrate 501 has a pattern of squares 502 separated bybreak-off trenches 503. By way of example, the substrate 501 is made ofpolyester, the squares 502 are squares of 20×20 micron with a logo“JDSU” impressed therein for the purpose of tracking orientation of thesquares in a coating. The substrate 501 may have a different, notnecessarily square, pattern of tranches 503, as it will be discussedfurther with reference to FIGS. 9A-C.

The releasable coating formed during the coating step 530 includes oneor more magnetic layers and, optionally, non-magnetic layers as it willbe discussed further with reference to FIGS. 10 and 11. The non-magneticlayers may be deposited using any conventional thin film depositiontechniques. Non-limiting examples of such techniques include physicalvapor deposition (PVD), chemical vapor deposition (CVD), plasma enhanced(PE) variations thereof, such as PECVD or downstream PECVD, sputtering,electrolytic deposition, sol-gel, and other like deposition methods thatlead to the formation of substantially uniform continuous thin filmlayers. For deposition of the magnetic layer, these techniques areperformed as described below.

In one embodiment of the method, the coating step 530 includesdeposition of the magnetic layer using at least two sources forproviding magnetic material to a same portion of the substrate atdifferent angles.

By way of example, in one embodiment of the method, the coating step 530includes deposition of the magnetic layer using at least two sources forproviding magnetic material to a same portion of a moving substratesimultaneously at two, or more, different angles. FIGS. 4A and 4Bschematically illustrate deposition of a magnetic material on thesurface of a substrate 601 moving in a direction 602. The magneticmaterial is provided to the substrate 601 simultaneously from twomagnetic material sources 603 and 604 disposed so that a line 610connecting the magnetic material sources 603 and 604 is substantiallyorthogonal to the direction 602. A charge of the magnetic material isplaced into the magnetic material sources 603 and 604, or crucibles, andheated. As the temperature of the charge rises, its vapor pressureincreases also, resulting in a significant evaporation rate. The vaporof the magnetic material leaves the crucibles in two streams 605 andgoes predominantly toward the substrate 601. The streams 605 intersectin a zone 606 within the vacuum chamber in close proximity to thesubstrate 601. The evaporant condenses on the cold substrate 601producing a magnetic thin film layer 608 underneath the substrate 601.The two overlapping vapor streams 605 come to a portion of the substrate601 simultaneously at two different angles. It is our understanding thatthey provide nucleation and growth of the magnetic material creating amicrocrystalline structure of grains of the condensate such that themagnetic layer 608 acquires an in-plane magnetic anisotropy in thedirection 602 orthogonal to the line connecting the evaporation sources603 and 604. The deposited removable coating including the magneticlayer 608 is then stripped off the substrate 601 and ground-broken alongthe trenches 503 shown in FIG. 3A into individual square-shaped flakes.As a result, a magnetic layer of an individual flake has amicrocrystalline or a domain structure providing a magnetic anisotropyoriented substantially parallel to one side of the square and orthogonalto another side of the square.

In one embodiment of the method, the coating step 530 includesdeposition of the magnetic layer illustrated in FIGS. 5A and 5B. Atleast two sources of magnetic material are used for providing themagnetic material to a same portion of the substrate at differentangles, wherein two or more sources of the magnetic material 103 and 104are disposed along the direction 102 of the substrate's movement andoriented at different angles with respect to the substrate 101, so thattwo evaporation streams 105 reach a portion of the substrate 101 atdifferent angles at different times.

In one embodiment of the method, the coating step 530 includesdeposition of the magnetic layer onto a static substrate as illustratedin FIGS. 6A and 6B. Two or more sources of the magnetic material 203 and204 create overlapping vapor streams 205 reaching the substrate 201 in aregion 206 so as to provide magnetic material to a same portion of thesubstrate at two different angles. Alternatively, the substrate 201shown in FIGS. 6A and 6B can move.

In one embodiment of the method, the substrate is moving during thedeposition of magnetic layer in the coating step 530, preferably in thedirection of the desired magnetic anisotropy.

In one embodiment of the method, the coating step 530 includesdeposition of the magnetic material onto a curved substrate asillustrated in FIG. 7. The substrate 701, which is partially wrappedaround a roller 702 inside of a roll-coater. A stream of the magneticmaterial 703 comes from a source of the magnetic material 704 such as ane-gun. A shield 705 prevents deposition of the material on the curvedsubstrate 701 before it comes to the roller 702. Because of thesubstrate's curved surface, a same portion of the substrate 701 movingthrough the evaporated stream 703 receives the magnetic material atsubstantially different angles. It has been proven experimentally, thata layer of the magnetic material deposited on the substrate 701 has amagnetic anisotropy oriented along the direction of the substrate'smovement, since flakes made by removing a releasable coating from thesubstrate and breaking it apart exhibited side-by-side alignment whensubjected to an external magnetic field.

Of course, features of different embodiments described herein can becombined. By way of example, the method embodiment shown in FIG. 4A mayuse the substrate 601 bent as shown in FIG. 7.

In one embodiment of the method, the coating step 530 includes annealingof the magnetic material in a magnetic field for providing an anisotropyin a desired direction. In another embodiment, the ion bombardment ofthe freshly deposited magnetic layer used for the same purpose. By wayof example, Xe-ion-irradiation of the magnetic material taught by KunZhang in “Stress induced magnetic anisotropy of Xe-ion-irradiated Nithin films”, Nucl. Instr. And Meth. In Phys. Res., B 243 (2006), 51-57,incorporated herein by reference. The bombardment causes physicalchanges of the microstructure structure of the magnetic materialproducing strain.

The method described herein with reference to FIG. 3B provides magneticflakes having a predefined two-dimensional shape and a magneticanisotropy oriented in a desired direction, which may be different fromthe direction of the longest planar dimension of the shape. Anisotropicmagnetic flakes of the instant invention have a thickness in the rangeof 50 nm to 10 microns and a two-dimensional shape 900 having adiameter, i.e. a greatest distance between two points of the shape 900,in the range of 1-500 microns.

In one embodiment of the instant invention, the flakes manufacturedusing the aforedescribed method are non-grated, reflective, anisotropic,magnetic flakes for forming a reflective coating. The flakes have aparticular shape and a magnetic anisotropy so as to enable formingribbons when dispersed in a liquid carrier and impacted with an externalmagnetic field. The flakes include two reflector layers and a magneticlayer therebetween, and have a smooth surface absent of a grating,however indicia may be present. The indicia may include symbols, logos.etc. Preferably, the indicia is symmetrical with respect to a directionof the in-plane magnetic anisotropy. By way of example, letters “B, “C,”“D” and numerals “3” and “8” have a horizontal symmetry axis.Anisotropic magnetic flakes with such letters, where a direction ofmagnetic anisotropy coincides with the direction of the horizontalsymmetry axis, form ribbons with side-by-side letters. The letters areeasily readable under magnification without turning a substratesupporting the ribbons or changing the observation angle. Letters “T,”“A,” “H,” “W,” “V,” “O,” etc., have a vertical symmetry axis. If themagnetic anisotropy of the flakes coincides with the vertical symmetryaxis, the letters on the ribbons are easily readable one letter underanother. In a coating comprising different flakes having an indicia, itis preferable to have a majority of the flakes being anisotropicmagnetic flakes with a symmetrical indicia, all having a same, verticalor horizontal symmetry axis, so as to form ribbons easily identifiableon a document. Preferably, the amount of anisotropic flakes with asymmetrical indicia is at least 70% of all the flakes, and morepreferably at least 90%. In one embodiment, a coating containsanisotropic flakes with asymmetrical indicia, such that the flakes havea hydrophobic coating on one side of the flake as taught in U.S. PatentApplication Publication No. US 2008/0233401 for desirable orientation ofthe flakes; ribbons of such flakes are also easily readable undermagnification without turning the document.

With reference to FIGS. 9A-C, the flakes have a two-dimensional shapewith two sides substantially parallel to one another, for example asquare 930 has two parallel sides 938. The flakes are manufactured so asto have a magnetic anisotropy in a direction 932 substantiallyorthogonal to the two mutually parallel sides 938. When such flakes aredispersed in a liquid carrier and impacted with an external magneticfield, they form ribbons such as shown in FIG. 2.

Preferably, the flakes have a square shape, but rectangles with unequalsides, parallelograms 900, hexagons 940, octagon 950, and any othershape having two sides substantially parallel to one another, aresuitable for forming ribbons. The direction of the magnetic anisotropy932, 942, or 952, forms with the longest planar dimension 934, 944, or954, respectively, an angle of at least 20 degrees.

Depending on the two-dimensional shape, some flakes would require ahydrophobic coating for turning up the right surface of the flake astaught in U.S. application Ser. No. 12/051,164 filed Mar. 19, 2008, 241,incorporated herein by reference. By way of example,parallelogram-shaped magnetic flakes 900 with a hydrophobic coating onone surface of the flake form better ordered ribbons shown in FIG. 9Bthan flakes without such a coating, as shown in FIG. 9C.

The magnetic layer can be formed of any magnetic material, such asferromagnetic and ferrimagnetic materials, including nickel, cobalt,iron, gadolinium, terbium, dysprosium, erbium, and their alloys oroxides. For example, a cobalt nickel alloy can be employed, with thecobalt and nickel having a ratio by weight of about 80% and about 20%,respectively. This ratio for each of these metals in the cobalt nickelalloy can be varied by plus or minus about 10% and still achieve thedesired results. Thus, cobalt can be present in the alloy in an amountfrom about 70% to about 90% by weight, and nickel can be present in thealloy in an amount from about 10% to about 30% by weight. Other examplesof alloys include Fe/Si, Fe/Ni, FeCo, Fe/Ni/Mo, and combinationsthereof. Hard magnetics of the type SmCo5, NdCo5, Sm2Co17, Nd2Fe14B,Sr6Fe2O3, TbFe2, Al—Ni—Co, and combinations thereof, can also be used aswell as spinel ferrites of the type Fe3O4, NiFe2O4, MnFe2O4, CoFe2O4, orgarnets of the type YIG or GdIG, and combinations thereof. The magneticmaterial may be selected for its reflecting or absorbing properties aswell as its magnetic properties. When utilized to function as areflector, the magnetic material is deposited to a thickness so that itis substantially opaque. When utilized as an absorber, the magneticmaterial is deposited to a thickness so that it is not substantiallyopaque. A typical thickness for the magnetic material when utilized asan absorber is from about 2 nm to about 20 nm.

The magnetic layer may be formed by a material having magnetic andnon-magnetic particles, or magnetic particle within non-magnetic medium,for example cobalt-doped zinc oxide film deposited using the sol-geltechnology.

Although this broad range of magnetic materials can be used, the “soft”magnets are preferred. As used herein, the term “soft magnets” refers toany material exhibiting ferromagnetic properties but having a remanencethat is substantially zero after exposure to a magnetic force. Softmagnets show a quick response to an applied magnetic field, but havevery low (coercive fields (Hc)=0.05-300 Oersteds (Oe)) or zero magneticsignatures, or retain very low magnetic lines of force after themagnetic field is removed. Similarly, as used herein, the term “hardmagnets” (also called permanent magnets) refers to any material thatexhibits ferromagnetic properties and that has a long lasting remanenceafter exposure to a magnetizing force. A ferromagnetic material is anymaterial that has a permeability substantially greater than 1 and thatexhibits magnetic hysteresis properties.

Preferably, the magnetic materials used to form magnetic layers in theflakes and foils of the invention have a coercivity of less than about2000 Oe, more preferably less than about 300 Oe. Coercivity refers tothe ability of a material to be demagnetized by an external magneticfield. The higher the value of coercivity, the higher the magnetic fieldrequired to de-magnetize the material after the field is removed. Themagnetic layers used are preferably “soft” magnetic materials (easilydemagnetized), as opposed to “hard” magnetic materials (difficult todemagnetize) which have higher coercivities. The coercivities of thefoils, pigments or colorants of the magnetic color shifting designsaccording to the invention are preferably in a range of about 50 Oe toabout 300 Oe. These coercivities are lower than in standard recordingmaterials. The use of soft magnetic materials in pigment flakes allowsfor easier dispersion of the flakes without clumping.

The magnetic layer can be formed to have a suitable physical thicknessof from about 200 angstroms to about 10,000 angstroms, and preferablyfrom about 500 to about 1,500 angstroms. However, it will be appreciatedby those skilled in the art, in view of the disclosure herein, that theoptimal magnetic thickness will vary depending on the particularmagnetic material used and the purpose for its use.

Anisotropic magnetic flakes have one or more substantially continuousthin-film layers, including a magnetic layer having a magneticanisotropy oriented as discussed above. Optical design of the flakes canbe different. The flakes can be silver-like with one of the followingstructures: M, R/M, R/M/R, M/D/M, M/D/R, D/R/M/R/D (where M is magneticmetal, R is reflective material and D is supportive or dielectricmaterial) or any other combination of a magnetic layer, reflective layerand a supportive layer. With reference to FIG. 10, the anisotropicreflective magnetic flakes (RMF) 20 include two reflector layers 24 and26 on both major surfaces of a magnetic layer 22 having anisotropy asdiscussed above. The reflector layers 24 and 26 can be composed ofvarious reflective materials. Presently preferred materials are one ormore metals, one or more metal alloys, or combinations thereof, becauseof their high reflectivity and ease of use, although non-metallicreflective materials could also be used. Nonlimiting examples ofsuitable metallic materials for the reflector layers include aluminum,silver, copper, gold, platinum, tin, titanium, palladium, nickel,cobalt, rhodium, niobium, chromium, and combinations or alloys thereof.The reflector layers can be formed to have a suitable physical thicknessof from about 40 to about 2,000 nm, and preferably from about 60 toabout 1,000 nm. The reflector layers have a reflectivity of at least 40%and preferably higher than 60%.

Optionally, the anisotropic reflective magnetic flakes include twoprotective layers disposed on the reflector layers, not shown in FIG.10; the protective layers are dielectric layers formed of such materialsas zinc sulfide (ZnS), zinc oxide (ZnO), zirconium oxide (ZrO2),titanium dioxide (TiO2), diamond-like carbon, indium oxide (In2O3),indium-tin-oxide (ITO), tantalum pentoxide (Ta2O5), ceric oxide (CeO2),yttrium oxide (Y2O3), europium oxide (Eu2O3), iron oxides such as(II)diiron(III) oxide (Fe3O4) and ferric oxide (Fe2O3), hafnium nitride(HfN), hafnium carbide (HfC), hafnium oxide (HfO2), lanthanum oxide(La2O3), magnesium oxide (MgO), neodymium oxide (Nd2O3), praseodymiumoxide (Pr6O11), samarium oxide (Sm2O3), antimony trioxide (Sb2O3),silicon monoxide (SiO), selenium trioxide (Se2O3), tin oxide (SnO2),tungsten trioxide (WO3), combinations thereof, and the like.

Optionally, the anisotropic reflective magnetic flakes of the instantinvention are color-shifting flakes 300 shown in FIG. 11, including anRMF 342 formed of a magnetic layer and two reflector layers on twosurfaces of the magnetic layer, two dielectric layers 344 and 346supported by the reflector layers of the RMF 342, and two absorberlayers 348 and 350 supported by the dielectric layers 344 and 346, forproviding a color-shifting optical effect. Materials suitable for themagnetic, reflector and dielectric layers are the same as listed abovewith reference to FIG. 10. The coatings formed using the color-shiftingflakes 300 are highly-reflective, color-shifting coatings with ribbonsformed therein.

Nonlimiting examples of suitable absorber materials include metallicabsorbers such as chromium, aluminum, nickel, silver, copper, palladium,platinum, titanium, vanadium, cobalt, iron, tin, tungsten, molybdenum,rhodium, and niobium, as well as their corresponding oxides, sulfides,and carbides. Other suitable absorber materials include carbon,graphite, silicon, germanium, cermet, ferric oxide or other metaloxides, metals mixed in a dielectric matrix, and other substances thatare capable of acting as a uniform or selective absorber in the visiblespectrum. Various combinations, mixtures, compounds, or alloys of theabove absorber materials may be used to form the absorber layers offlake 300.

Examples of suitable alloys of the above absorber materials includeInconel (NiCr—Fe), stainless steels, Hastalloys (e.g., Ni—Mo—Fe;Ni—Mo—Fe—Cr; Ni—Si—Cu) and titanium-based alloys, such as titanium mixedwith carbon (Ti/C), titanium mixed with tungsten (Ti/W), titanium mixedwith niobium (Ti/Nb), and titanium mixed with silicon (Ti/Si), andcombinations thereof. As mentioned above, the absorber layers can alsobe composed of an absorbing metal oxide, metal sulfide, metal carbide,or combinations thereof. For example, one preferred absorbing sulfidematerial is silver sulfide. Other examples of suitable compounds for theabsorber layers include titanium-based compounds such as titaniumnitride (TiN), titanium oxynitride (TiNxOy), titanium carbide (TiC),titanium nitride carbide (TiNxCz), titanium oxynitride carbide(TiNxOyCz), titanium silicide (TiSi2), titanium boride (TiB2), andcombinations thereof In the case of TiNxOy and TiNxOyCz, preferably x=0to 1, y=0 to 1, and z=0 to 1, where x+y=1 in TiNxOy and x+y+z=1 inTiNxOyCz. For TiNxCy, preferably x=0 to 1 and z=0 to 1, where x+z=1.Alternatively, the absorber layers can be composed of a titanium-basedalloy disposed in a matrix of Ti, or can be composed of Ti disposed in amatrix of a titanium-based alloy.

Optionally, a ribbon-forming anisotropic magnetic flake has a magneticlayer, by way of example, formed of nickel, having a shape andanisotropy as discussed above, which does not support any reflectorlayers. However, at least one reflector layer is desirable for providinga higher reflectivity of the coating.

To compare magnetic properties of conventional flakes and flakes of theinstant invention, two types of flakes have been manufactured.

Using a conventional method, a web substrate patterned as shown in FIG.3, was coated with a releasable thin-film coating MgF₂/Al/Ni/Al/MgF₂ ina batch coater. The deposited coating was stripped off the substrate andground until all particles were broken along break-off trenches 503. Theresulting flakes were mixed with transparent UV-curable ink vehicle inconcentration of 20 wt % to produce a magnetic pigment. The ink wasscreen-printed on a paper card and cured in UV light. A microscopicimage in FIG. 8A illustrates a random alignment of the flakes.Additionally. flakes of the same type were mixed with the ink vehicle ina smaller concentration (2 wt %), printed onto another paper card andinserted between two permanent magnets 801 and 802 as illustrated inFIG. 8B, wherein the flakes aligned to have their diagonalssubstantially along the field direction 803.

Non-grated, anisotropic, reflective, magnetic flakes were manufacturedusing a method of the instant invention illustrated in FIGS. 4A and 4B,wherein the substrate 601 was arched in a roll coater as shown in FIG.7. A releasable coating MgF₂/Al/Ni/Al/MgF₂ have been deposited on thesubstrate, then stripped off the substrate and ground until allplatelets were broken into squares. The resulting flakes were mixed withthe same ink vehicle in the concentration of 10 wt % and printed on apaper card. As illustrated in FIG. 8C, a wet print containing magneticflakes was placed between two magnets 801 and 802. The direction of anapplied magnetic field is indicated by the arrow 803. The flakes, inaccordance with the instant invention manufactured to have a magneticmoment in the direction of one side of the square, aligned with theirsides parallel to the direction of applied magnetic field 803. Anincrease of the flake concentration allows their assembly in long ribbonshown in FIG. 8D. A further increase of concentration brings the flakesclose to each other (FIG. 8E) forming a highly reflective coating on apaper card.

The non-grated, anisotropic, reflective, magnetic flakes of the instantinvention, also referred herein as reflective flakes, are designed forforming a highly reflective coating, in particular desirable in theprinting industry. A conventional method of increasing reflectivity of ametallic pigment includes surface modification of a metal flake bysurfactants, such as fatty acids. The surfactants reduce the surfaceenergy of the flake and make it float to the surface of the coating.However, the surfactants substantially reduce the abrasion wear of thecoating. Magnetically oriented square flakes, assembled in long ribbonsand often extending across the entire printed insignia parallel to thesurface of the ink, fully utilize the total reflective surface of thepigment. Optionally, the coating provides a color-shifting effect if theflakes are color-shifting anisotropic reflective magnetic flakes. Thereflective flakes have a substantially smooth surface for providing highspecular reflectivity of the coating. The flakes have no grating, butmay have indicia thereon.

It has been taught in US Patent Application No. 20060263539 publishedNov. 23, 2006, to Argoitia, which is incorporated herein by reference,that magnetic flakes with a diffractive grating or a magnetic layerformed of separate stripes orient so as to align grating grooves orstripes along the lines of the applied magnetic field. However, it hasbeen not known so far how to align smooth-surface flakes having acontinuous magnetic layer so as to have a side parallel to a directionof an external magnetic field. Also, it has not been known how toassemble flakes in long, flat ribbons of equal width.

To form a reflective coating, a carrier and a plurality of flakesdispersed therein are provided to a surface of an object and then amagnetic field is applied for orientation of the flakes parallel to thesurface of the coating.

Carriers are typically liquid for a period to permit some motion of theflake before the carrier evaporates or hardens. For example, ink mighthave a volatile carrier that evaporates to fix the flake, or a clearpaint carrier, such as a clear paint base, might harden to fix theflake. Similarly, uncured thermosetting resin or heated thermoplasticresin might allow the flake to be oriented prior to curing or cooling,respectively, either before, during, or after application to a surface.By way of example, the carrier is an acrylic resin based carrier; othercarriers are readily known to one skilled in the art.

Examples of carriers include polyvinyl alcohol, polyvinyl acetatepolyvinylpyrrolidone, poly(ethoxyethylene), poly(methoxyethylene),poly(acrylic) acid, poly(acrylamide), poly(oxyethylene), poly(maleicanhydride), hydroxyethyl cellulose, cellulose acetate, poly(saccharides)such as gum arabic and pectin, poly(acetals) such as polyvinylbutyral,poly(vinyl halides) such as polyvinyl chloride and polyvinylenechloride, poly(dienes) such as polybutadiene, poly(alkenes) such aspolyethylene, poly(acrylates) such as polymethyl acrylate,poly(methacrylates) such as poly methylmethacrylate, poly(carbonates)such as poly(oxycarbonyl oxyhexamethylene, poly(esters) such aspolyethylene terephthalate, poly(urethanes), poly(siloxanes),poly(suphides), poly(sulphones), poly(vinylnitriles),poly(acrylonitriles), poly(styrene), poly(phenylenes) such as poly(2,5dihydroxy-1,4-phenyleneethylene), poly(amides), natural rubbers,formaldahyde resins, other polymers and mixtures of polymers, polymerswith solvents, as well as photopolymers.

To ensure that a coating has a high reflectivity of at least 40%, theflakes have reflector layers having reflectivity of greater than 50%and, preferably, greater than 60%, and the flakes have a specific shapeand magnetic anisotropy for tiling a surface of the coated object withflakes adjacent to one another substantially leaving no surface open,provided a concentration of the flakes is high enough.

Due to the specific shape and magnetic anisotropy, a portion of theflakes attract to each other side-by-side and form one or more ribbonsas discussed above with reference to FIG. 2 and shown in FIG. 8D. Theribbons are understood to have at least three flakes adjacentside-by-side with gaps between the flakes of no greater than 500 nm. Nogaps were noticed for the flakes in the size range of 1-20 microns. Whenthe flakes have a size in the range of 20-500 microns, the gap sizevaries from zero to 500 nm. Of course, formation of the ribbons dependson a concentration of the flakes in the carrier.

To form a highly reflective coating, almost all of the object's surfaceunder the coating should be covered with reflective flakes leaving no orlittle space between the flakes where the coating has lower localreflectivity corresponding to reflectivity of the carrier on theobject's surface in absence of reflective flakes. Accordingly, anaggregate surface of the flakes, i.e. a sum of all flake surfaces turnedto an observer, is equal to at least 80% of the surface's area under thecoating. Preferably, the aggregate surface of the flakes is greater than90% of the surface's area under the coating. Such concentration of theflakes provides substantially total coverage of the object's surfacewith ribbons of the flakes forming substantially a tile array of flakesshown in FIG. 8E. The tile array is understood as including at least tworibbons adjacent to one another so as to be side-by-side with gapsbetween the ribbons of no greater than 4 microns.

To provide such coverage using conventional flakes, a thick coating withmultiple levels of flakes is required so that flakes of a next levelpartially cover gaps between randomly dispersed flakes of previouslevels. Additionally, a high concentration of flakes is associated ahigher probability of flakes overlapping and with a higher cost of thecoating. Advantageously, the flakes of the instant invention provide athin, cost-effective, highly-reflective coating. Furthermore, flakesaligned in a ribbon provide a security feature to the object with noextra cost or effort. The ribbons can be used for authentication of theobject. A conventional image recognition technique applied to the imagereflected by the coating so as to indentify whether any ribbons arepresent therein.

In one embodiment, anisotropic, reflective, magnetic flakes with anon-periodic linear grating are dispersed in a carrier for forming acoating with well-defined ribbons, which may be used as a securityfeature. The flakes with a non-periodic linear grating have the samelayered structure and two-dimensional shape as the non-diffractive,anisotropic, reflective, magnetic flakes described above. The flakes ofthis embodiment may be reflective or a color-shifting, they have anon-symmetric and non-periodic structure of the grating that reducespresence of diffractive colors.

A grating is any regularly spaced collection of essentially identical,parallel, elongated elements. In some instances, the grating can benon-periodic non-regularly spaced collection of non-identical parallelelongated elements. Gratings can be diffractive, holographic,reflective, binary, etc. A grating can also be a picture havingcharacteristics of a grating. Holographic gratings are widely used forfabrication of holograms in packaging industry, for securityapplications and in the art. Diffractive gratings are also used forpackaging. Diffractive flakes are fabricated by deposition of an opticalstack onto a surface of a substrate with a diffractive grating. Theseflakes provide diffractive colors when illuminated by light. Magneticdiffractive flakes orient themselves with their grating parallel to thedirection of applied magnetic field. Being dispersed in ink and alignedin the magnetic field, the shapeless diffractive flakes of differentsizes form chains similar to one shown in FIG. 1B.

Magnetic flakes with reflective gratings are illustrated in FIG. 12. Thewidths of the elements of the grating are not equal to each other:W₁≠W₂≠W₃≠W₄≠W₅ as well as their heights: H₂≠H₃. A single pigment flakeneeds just a few (usually from one to four) grooves to provide flakeorientation in the field. The size of the flake can be in the range of2×2 microns to 200×200 microns. The heights (FIG. 12) of the gratingelements can be in the range of 10 nm to 100 nm. The widths of thegrating elements can be in the range of 10 nm to 90 nm. Rectangularmagnetic reflective grating flakes, all having a same shape, beingdispersed in an ink vehicle and exposed to an external magnetic field,orient themselves along the direction of the field and assemble in longribbon-like structures of an equal width as shown in FIG. 13.

The direction of the magnetic anisotropy is the direction of thenon-periodic grating. Therefore, for forming ribbons in a coating, theanisotropic, reflective, magnetic flakes with a non-periodic gratinghave a two-dimensional shape with two sides substantially parallel toeach other, and a non-periodic grating substantially orthogonal to thetwo sides.

A thin film composition MgF₂/Al/Ni/Al/MgF₂ was deposited on the top ofstructured substrate having embossed patterns of 20×20 micron squares.Every square consisted of four flat embossed elements (hills) and threenarrow debossed (valleys) elements. The widths of the embossed anddebossed elements were different to reduce the diffractive component ofthe light reflected from the flake. The coated structure was releasedfrom the substrate and ground. With reference to FIG. 13, the flakeswere mixed with a transparent ink vehicle 470, silk screen printed on atransparent polyester card and exposed to the parallel magnetic field.The ink composition was solidified with UV lamp and microscopicallyanalyzed. As shown in FIG. 13, square magnetic reflective flakes 480form continuous, well-defined ribbons of constant width, often extendingfrom one side of the printed coating to another.

FIG. 14 illustrates a method of authentication of an object such as abank note 430, which has a printed coating 420. The coating 420 includesdiffractive, anisotropic, reflective, magnetic flakes with a binary(fanout) grating disclosed in U.S. application Ser. No. 12/107,152 filedApr. 22, 2008, incorporated herein by reference. The flakes are coatedwith an oleophobic/hydrophobic material so as to be aligned on thesurface of the ink. For identifying a ribbon and a pattern provided bythe binary grating, a monochromatic beam of light 410 from a laser 400illuminates the printed coating 420 containing ribbons. Reflected beams440, forming a predetermined pattern, are received at an image sensor orCCD camera 450 connected to a computer 460 or a reader. The computer 460decodes the pattern so as to authenticate the bank note 430.

We claim:
 1. An anisotropic magnetic flake comprising: a two-dimensionallayered structure comprising: a magnetic layer, where the magnetic layerhas a structure that provides in-plane magnetic anisotropy, where adirection of the in-plane magnetic anisotropy is substantiallyorthogonal to two substantially parallel sides, of the two-dimensionallayered structure, for alignment of the anisotropic magnetic flakerelative to one or more other flakes, and where a gap between theanisotropic magnetic flake and a flake, of the one or more other flakes,is no greater than 500 nm when the anisotropic magnetic flake and theflake are under influence of an external magnetic field.
 2. Theanisotropic magnetic flake of claim 1, where the one or more otherflakes have a same structure as the anisotropic magnetic flake.
 3. Theanisotropic magnetic flake of claim 1, where the anisotropic magneticflake is aligned side-by-side with the one or more other flakes.
 4. Theanisotropic magnetic flake of claim 1, where the magnetic layer has afirst surface and a second surface, and where the two-dimensionallayered structure further comprises: a first reflector layer disposed onthe first surface of the magnetic layer; and a second reflector layerdisposed on the second surface of the magnetic layer.
 5. The anisotropicmagnetic flake of claim 4, where the magnetic layer comprises a magneticmaterial having a coercivity of less than about 2000 Oe.
 6. Theanisotropic magnetic flake of claim 1, where the two-dimensional layeredstructure includes two first sides parallel to each other and two secondsides parallel to each other.
 7. The anisotropic magnetic flake of claim1, where the in-plane magnetic anisotropy is oriented in a directionthat is at an angle of at least 20 degrees relative to a longest planardimension of the two-dimensional layered structure.
 8. The anisotropicmagnetic flake of claim 1, where the magnetic layer is a continuousmagnetic layer absent of a grating.
 9. The anisotropic magnetic flake ofclaim 1, where the magnetic layer aligns the anisotropic magnetic flakesubstantially parallel to a surface of a reflective coating when theanisotropic magnetic flake is disposed in a liquid carrier and underinfluence of an external magnetic field.
 10. A flake comprising: atwo-dimensional layered structure comprising: a magnetic layer, and areflector layer disposed on a surface of the magnetic layer, wherein thetwo-dimensional layered structure includes two first sides parallel toeach other and two second sides parallel to each other, where themagnetic layer has a structure that provides magnetic anisotropy, wherethe flake is aligned relative to one or more other flakes, and where agap between the flake and another flake, of the one or more otherflakes, is no greater than 500 nm when the anisotropic magnetic flakeand the flake are under influence of an external magnetic field.
 11. Theflake of claim 10, where the two-dimensional layered structure furthercomprises: a different reflector layer disposed on a different surfaceof the magnetic layer, and where the magnetic layer is between thereflector layer and the different reflector layer.
 12. The flake ofclaim 10, where the two-dimensional layered structure has a thickness ina range of 50 nm to 10 microns.
 13. The flake of claim 10, where themagnetic anisotropy is oriented in a direction that is different from adirection of a longest planar dimension of the two-dimensional layeredstructure.
 14. The flake of claim 10, where the flake is non-grated. 15.The flake of claim 10, where the flake is reflective.
 16. The flake ofclaim 10, where a diameter of the flake is in a range of 1 to 500microns.
 17. The flake of claim 10, where the flake has an indicia thatis symmetrical with respect to a direction of the magnetic anisotropy.18. A flake comprising: a first reflector layer; a second reflectorlayer; and a magnetic layer between the first reflector layer and thesecond reflector layer, where the flake is aligned relative to one ormore other flakes, and where a gap between the flake and another flake,of the one or more other flakes, is no greater than 500 nm when theanisotropic magnetic flake and the flake are under influence of anexternal magnetic field.
 19. The flake of claim 18, where the flake hasat least two sides parallel to each other.
 20. The flake of claim 18,where the magnetic layer has a structure that provides magneticanisotropy.